1. Field of the Invention
The present invention relates to a color image picture forming process and apparatus for forming a color image picture using coloring materials (colorants) of four colors including three primary colors of cyan, magenta and yellow or the like, and black and by adhering each colorant on a paper corresponding to one picture element (pixel).
In more particular, the present invention relates to a method of using the colorant black.
2. Description of the Prior Art
In ink-jet printers and the like, it is theoretically possible to reproduce black using three primary colors of cyan, magenta and yellow as a subtractive mixture; however, it is in practice impossible to express black with high density due to the blot by overlapping the inks of three colors.
Therefore, in general, black is further added to the three colors of cyan, magenta and yellow to obtain four colors and the black component (colorless component) is replaced by black, thereby constituting an image picture.
For example, assuming that the densities of cyan, magenta and yellow of a certain pixel are c, m and y, respectively, there is adopted such a method that the density of the black ink is determined such that, for example, BK=min (y, m, c) and in the case where this black component level BK is larger than a certain constant threshold, the use of the black ink is started.
In such a method, since the lowest reflection optical density which can be expressed by monochrome of black is fairly high, the use of the black ink is suddenly started from a density point more than a certain density. Namely, the use of black is suddenly started from a density point more than a certain density in the portion which was a smooth shadow portion in the original image picture. Consequently, an unnatural false contour or "pseudo profile" would have been produced.
In addition, since qualities of the colorants to be used such as inks, toner, etc. generally increase in the high-density portion of an image picture, the gradient information in the image picture will be lost due to the flow and blot of the inks, causing picture quality to deteriorate.
Particularly in case of a color image picture, even if under color removal is performed, since a high quantity of ink is still adhered in the high-density portion, this also causes picture quality to deteriorate.
For instance, assuming that a human face is the intermediate gradient image picture, since black hair is expressed as substantially the high-density portion, a flat image will be obtained, so that the impression of hair is lost and there occurs a problem of deterioration of quality of the whole image picture.
On the other hand, in color printers, black is expressed using the black colorant in addition to the three primary colors as described above. However, the contrast of the whole image picture is determined depending upon the highest OD value (reflection optical density) to be expressed by the black colorant; therefore, if one desires to express from the low OD value to the high OD value by the black colorant, it is difficult to express high OD values and the contrast of the image picture is not satisfactorily derived.